The invention relates to weighing scales in general, and more particularly to improvements in electromechanical weighing scales of the type wherein a deformable carrier supports one or more resistors or semiconductors serving to generate signals which are indicative of the load that is applied to the load receiving and transmitting member of the scale, e.g., to a platform forming part of a bathroom scale.
In accordance with a presently known proposal, the entire platform of a bathroom scale is made of a relatively stiff metallic sheet material and each corner of the platform is disposed above a strain gauge whose resistor changes its resistance in response to the application of a load to the upper side of the platform. The carriers of resistors in the strain gauges transmit forces to the base of the scale. The strain gauges operate independently of each other, and the circuit of the scale comprises means for totalizing the signals which are transmitted by the resistors of discrete strain gauges to thus ascertain the overall load, e.g., the weight of a person standing on the platform. Reference may be had, for example, to published European patent application No. 0 034 656 of Lockery.
Such scale is expensive because it must employ a substantial number (normally four) of strain gauges.
In accordance with another known proposal, the scale comprises a relatively stiff platform which is designed to transmit the load to a rigid base. The latter has four legs which transmit the load to the ground, e.g., to the floor of a bathroom. One end portion of a single strain gauge is secured to the underside of the platform by a first set of screws, and the other end portion of such single strain gauge is affixed to the base by a second set of screws. The base is located at a first level, the platform is located at a second level above the first level, and the strain gauge is disposed at a third level between the first and second levels. A drawback of this scale is that the combined height of the base, strain gauge and platform is excessive.
It was further proposed to employ in a weighing scale strain gauges wherein the carriers of resistors have a substantially I-shaped cross-sectional outline and are mounted in such a way that they are subjected to shearing stresses in response to the application of a load to the platform. One end of each carrier is anchored in a support, and the other end of each carrier receives a load which is transmitted to the median portion of the carrier and to the resistor which is associated therewith. The load which is applied to the other end of the carrier can be transmitted by way of and in the longitudinal direction of the carrier without adversely influencing the characteristics of the signal from the resistor, even if the distance from the other end portion of the carrier to the resistor varies within a rather wide range. In the first approximation, a load which is applied eccentrically of the longitudinal axis of the carrier does not affect the accuracy of the signal from the corresponding resistor. However, the eccentricity cannot exceed a relatively small fraction of one centimeter which does not suffice in actual practice.
Another drawback of many presently known weighing scales, especially bathroom scales, is that their space requirements are excessive, especially if such scales are to be carried along on trips or are to be put in storage when not in actual use. This has led to attempts to provide collapsible weighing scales, for example, of the type disclosed in German Offenlegungsschrift No. 31 40 483 of Wirtz who discloses a scale wherein the base and the platform are articulately connected to each other by hinges so that the scale can be converted into a flat panel-like body by pivoting the platform into the plane of the base. When the scale is to be put to use, the platform is pivoted to a position of overlap with the base. A drawback of this scale is that each of its two parts must be individually calibrated to ensure proper suppression of so-called corner load errors. In fact, each such part constitutes a discrete scale and the apparatus must employ not less than four discrete strain gauges. This contributes significantly to the cost of the apparatus, especially since each of the two individual scales necessitates accurate calibration in order to ensure that the sensitivity of the scale including the platform will match the sensitivity of the scale which includes the base. As a rule, the strain gauges employ expensive piezoelectric elements. If such expensive elements are replaced with primitive piezoelectric foils or with polycrystalline die-formed or pressed parts, the strain gauges exhibit a highly unsatisfactory linearity, reproducibility and accuracy, especially over longer periods of time. All in all, such scales failed to gain acceptance because they lack the accuracy which is expected from a bathroom scale or from a like weighing apparatus.
German Offenlegungsschrift No. 36 42 970 of Iida et al. discloses a further collapsible scale wherein two platforms (one for each foot of a person standing on the scale) are coupled to each other by a hinge and each platform forms part of a discrete scale. Each discrete scale must be individually calibrated to ensure that the scales will exhibit identical sensitivities and that the compensation for corner loads in one of the scales will match that for corner loads of the other scale. Each of the two scales has a discrete carrier for a resistor, and each carrier is an elongated elastic strip of metallic sheet material. The end portions of the strip are disposed between pairs of rod-shaped deformation applying members which transmit torque in response to the application of a load to the respective platform. The inventors named in this published German patent application presume that the relatively wide strip-shaped carriers will be in a position to integrate eccentrically applied loads. A drawback of the proposal of Iida et al. is that the collapsible scale is expensive and that its accuracy is far from satisfactory. One of the reasons for lack of accuracy is that the aforementioned rod-shaped members act not unlike knife edge bearings and their mounting is not sufficiently accurate to ensure that they will remain in predetermined positions relative to each other.